# Source file src/time/time.go

```     1  // Copyright 2009 The Go Authors. All rights reserved.
2  // Use of this source code is governed by a BSD-style
4
5  // Package time provides functionality for measuring and displaying time.
6  //
7  // The calendrical calculations always assume a Gregorian calendar, with
8  // no leap seconds.
9  //
10  // # Monotonic Clocks
11  //
12  // Operating systems provide both a “wall clock,” which is subject to
13  // changes for clock synchronization, and a “monotonic clock,” which is
14  // not. The general rule is that the wall clock is for telling time and
15  // the monotonic clock is for measuring time. Rather than split the API,
16  // in this package the Time returned by time.Now contains both a wall
18  // operations use the wall clock reading, but later time-measuring
19  // operations, specifically comparisons and subtractions, use the
21  //
22  // For example, this code always computes a positive elapsed time of
23  // approximately 20 milliseconds, even if the wall clock is changed during
24  // the operation being timed:
25  //
26  //	start := time.Now()
27  //	... operation that takes 20 milliseconds ...
28  //	t := time.Now()
29  //	elapsed := t.Sub(start)
30  //
31  // Other idioms, such as time.Since(start), time.Until(deadline), and
32  // time.Now().Before(deadline), are similarly robust against wall clock
33  // resets.
34  //
35  // The rest of this section gives the precise details of how operations
36  // use monotonic clocks, but understanding those details is not required
37  // to use this package.
38  //
39  // The Time returned by time.Now contains a monotonic clock reading.
41  // both the wall clock and monotonic clock readings to compute the result.
42  // Because t.AddDate(y, m, d), t.Round(d), and t.Truncate(d) are wall time
43  // computations, they always strip any monotonic clock reading from their results.
44  // Because t.In, t.Local, and t.UTC are used for their effect on the interpretation
45  // of the wall time, they also strip any monotonic clock reading from their results.
46  // The canonical way to strip a monotonic clock reading is to use t = t.Round(0).
47  //
48  // If Times t and u both contain monotonic clock readings, the operations
49  // t.After(u), t.Before(u), t.Equal(u), and t.Sub(u) are carried out
50  // using the monotonic clock readings alone, ignoring the wall clock
51  // readings. If either t or u contains no monotonic clock reading, these
52  // operations fall back to using the wall clock readings.
53  //
54  // On some systems the monotonic clock will stop if the computer goes to sleep.
55  // On such a system, t.Sub(u) may not accurately reflect the actual
56  // time that passed between t and u.
57  //
58  // Because the monotonic clock reading has no meaning outside
59  // the current process, the serialized forms generated by t.GobEncode,
60  // t.MarshalBinary, t.MarshalJSON, and t.MarshalText omit the monotonic
61  // clock reading, and t.Format provides no format for it. Similarly, the
62  // constructors time.Date, time.Parse, time.ParseInLocation, and time.Unix,
63  // as well as the unmarshalers t.GobDecode, t.UnmarshalBinary.
64  // t.UnmarshalJSON, and t.UnmarshalText always create times with
65  // no monotonic clock reading.
66  //
67  // The monotonic clock reading exists only in Time values. It is not
68  // a part of Duration values or the Unix times returned by t.Unix and
69  // friends.
70  //
71  // Note that the Go == operator compares not just the time instant but
72  // also the Location and the monotonic clock reading. See the
73  // documentation for the Time type for a discussion of equality
74  // testing for Time values.
75  //
76  // For debugging, the result of t.String does include the monotonic
77  // clock reading if present. If t != u because of different monotonic clock readings,
78  // that difference will be visible when printing t.String() and u.String().
79  package time
80
81  import (
82  	"errors"
83  	_ "unsafe" // for go:linkname
84  )
85
86  // A Time represents an instant in time with nanosecond precision.
87  //
88  // Programs using times should typically store and pass them as values,
89  // not pointers. That is, time variables and struct fields should be of
90  // type time.Time, not *time.Time.
91  //
92  // A Time value can be used by multiple goroutines simultaneously except
93  // that the methods GobDecode, UnmarshalBinary, UnmarshalJSON and
94  // UnmarshalText are not concurrency-safe.
95  //
96  // Time instants can be compared using the Before, After, and Equal methods.
97  // The Sub method subtracts two instants, producing a Duration.
98  // The Add method adds a Time and a Duration, producing a Time.
99  //
100  // The zero value of type Time is January 1, year 1, 00:00:00.000000000 UTC.
101  // As this time is unlikely to come up in practice, the IsZero method gives
102  // a simple way of detecting a time that has not been initialized explicitly.
103  //
104  // Each Time has associated with it a Location, consulted when computing the
105  // presentation form of the time, such as in the Format, Hour, and Year methods.
106  // The methods Local, UTC, and In return a Time with a specific location.
107  // Changing the location in this way changes only the presentation; it does not
108  // change the instant in time being denoted and therefore does not affect the
109  // computations described in earlier paragraphs.
110  //
111  // Representations of a Time value saved by the GobEncode, MarshalBinary,
112  // MarshalJSON, and MarshalText methods store the Time.Location's offset, but not
113  // the location name. They therefore lose information about Daylight Saving Time.
114  //
115  // In addition to the required “wall clock” reading, a Time may contain an optional
116  // reading of the current process's monotonic clock, to provide additional precision
117  // for comparison or subtraction.
118  // See the “Monotonic Clocks” section in the package documentation for details.
119  //
120  // Note that the Go == operator compares not just the time instant but also the
121  // Location and the monotonic clock reading. Therefore, Time values should not
122  // be used as map or database keys without first guaranteeing that the
123  // identical Location has been set for all values, which can be achieved
124  // through use of the UTC or Local method, and that the monotonic clock reading
125  // has been stripped by setting t = t.Round(0). In general, prefer t.Equal(u)
126  // to t == u, since t.Equal uses the most accurate comparison available and
127  // correctly handles the case when only one of its arguments has a monotonic
129  type Time struct {
130  	// wall and ext encode the wall time seconds, wall time nanoseconds,
131  	// and optional monotonic clock reading in nanoseconds.
132  	//
133  	// From high to low bit position, wall encodes a 1-bit flag (hasMonotonic),
134  	// a 33-bit seconds field, and a 30-bit wall time nanoseconds field.
135  	// The nanoseconds field is in the range [0, 999999999].
136  	// If the hasMonotonic bit is 0, then the 33-bit field must be zero
137  	// and the full signed 64-bit wall seconds since Jan 1 year 1 is stored in ext.
138  	// If the hasMonotonic bit is 1, then the 33-bit field holds a 33-bit
139  	// unsigned wall seconds since Jan 1 year 1885, and ext holds a
140  	// signed 64-bit monotonic clock reading, nanoseconds since process start.
141  	wall uint64
142  	ext  int64
143
144  	// loc specifies the Location that should be used to
145  	// determine the minute, hour, month, day, and year
146  	// that correspond to this Time.
147  	// The nil location means UTC.
148  	// All UTC times are represented with loc==nil, never loc==&utcLoc.
149  	loc *Location
150  }
151
152  const (
153  	hasMonotonic = 1 << 63
154  	maxWall      = wallToInternal + (1<<33 - 1) // year 2157
155  	minWall      = wallToInternal               // year 1885
156  	nsecMask     = 1<<30 - 1
157  	nsecShift    = 30
158  )
159
160  // These helpers for manipulating the wall and monotonic clock readings
161  // take pointer receivers, even when they don't modify the time,
162  // to make them cheaper to call.
163
164  // nsec returns the time's nanoseconds.
165  func (t *Time) nsec() int32 {
167  }
168
169  // sec returns the time's seconds since Jan 1 year 1.
170  func (t *Time) sec() int64 {
171  	if t.wall&hasMonotonic != 0 {
172  		return wallToInternal + int64(t.wall<<1>>(nsecShift+1))
173  	}
174  	return t.ext
175  }
176
177  // unixSec returns the time's seconds since Jan 1 1970 (Unix time).
178  func (t *Time) unixSec() int64 { return t.sec() + internalToUnix }
179
181  func (t *Time) addSec(d int64) {
182  	if t.wall&hasMonotonic != 0 {
183  		sec := int64(t.wall << 1 >> (nsecShift + 1))
184  		dsec := sec + d
185  		if 0 <= dsec && dsec <= 1<<33-1 {
186  			t.wall = t.wall&nsecMask | uint64(dsec)<<nsecShift | hasMonotonic
187  			return
188  		}
189  		// Wall second now out of range for packed field.
190  		// Move to ext.
191  		t.stripMono()
192  	}
193
194  	// Check if the sum of t.ext and d overflows and handle it properly.
195  	sum := t.ext + d
196  	if (sum > t.ext) == (d > 0) {
197  		t.ext = sum
198  	} else if d > 0 {
199  		t.ext = 1<<63 - 1
200  	} else {
201  		t.ext = -(1<<63 - 1)
202  	}
203  }
204
205  // setLoc sets the location associated with the time.
206  func (t *Time) setLoc(loc *Location) {
207  	if loc == &utcLoc {
208  		loc = nil
209  	}
210  	t.stripMono()
211  	t.loc = loc
212  }
213
214  // stripMono strips the monotonic clock reading in t.
215  func (t *Time) stripMono() {
216  	if t.wall&hasMonotonic != 0 {
217  		t.ext = t.sec()
219  	}
220  }
221
222  // setMono sets the monotonic clock reading in t.
223  // If t cannot hold a monotonic clock reading,
224  // because its wall time is too large,
225  // setMono is a no-op.
226  func (t *Time) setMono(m int64) {
227  	if t.wall&hasMonotonic == 0 {
228  		sec := t.ext
229  		if sec < minWall || maxWall < sec {
230  			return
231  		}
232  		t.wall |= hasMonotonic | uint64(sec-minWall)<<nsecShift
233  	}
234  	t.ext = m
235  }
236
237  // mono returns t's monotonic clock reading.
238  // It returns 0 for a missing reading.
239  // This function is used only for testing,
240  // so it's OK that technically 0 is a valid
241  // monotonic clock reading as well.
242  func (t *Time) mono() int64 {
243  	if t.wall&hasMonotonic == 0 {
244  		return 0
245  	}
246  	return t.ext
247  }
248
249  // After reports whether the time instant t is after u.
250  func (t Time) After(u Time) bool {
251  	if t.wall&u.wall&hasMonotonic != 0 {
252  		return t.ext > u.ext
253  	}
254  	ts := t.sec()
255  	us := u.sec()
256  	return ts > us || ts == us && t.nsec() > u.nsec()
257  }
258
259  // Before reports whether the time instant t is before u.
260  func (t Time) Before(u Time) bool {
261  	if t.wall&u.wall&hasMonotonic != 0 {
262  		return t.ext < u.ext
263  	}
264  	ts := t.sec()
265  	us := u.sec()
266  	return ts < us || ts == us && t.nsec() < u.nsec()
267  }
268
269  // Equal reports whether t and u represent the same time instant.
270  // Two times can be equal even if they are in different locations.
271  // For example, 6:00 +0200 and 4:00 UTC are Equal.
272  // See the documentation on the Time type for the pitfalls of using == with
273  // Time values; most code should use Equal instead.
274  func (t Time) Equal(u Time) bool {
275  	if t.wall&u.wall&hasMonotonic != 0 {
276  		return t.ext == u.ext
277  	}
278  	return t.sec() == u.sec() && t.nsec() == u.nsec()
279  }
280
281  // A Month specifies a month of the year (January = 1, ...).
282  type Month int
283
284  const (
285  	January Month = 1 + iota
286  	February
287  	March
288  	April
289  	May
290  	June
291  	July
292  	August
293  	September
294  	October
295  	November
296  	December
297  )
298
299  // String returns the English name of the month ("January", "February", ...).
300  func (m Month) String() string {
301  	if January <= m && m <= December {
302  		return longMonthNames[m-1]
303  	}
304  	buf := make([]byte, 20)
305  	n := fmtInt(buf, uint64(m))
306  	return "%!Month(" + string(buf[n:]) + ")"
307  }
308
309  // A Weekday specifies a day of the week (Sunday = 0, ...).
310  type Weekday int
311
312  const (
313  	Sunday Weekday = iota
314  	Monday
315  	Tuesday
316  	Wednesday
317  	Thursday
318  	Friday
319  	Saturday
320  )
321
322  // String returns the English name of the day ("Sunday", "Monday", ...).
323  func (d Weekday) String() string {
324  	if Sunday <= d && d <= Saturday {
325  		return longDayNames[d]
326  	}
327  	buf := make([]byte, 20)
328  	n := fmtInt(buf, uint64(d))
329  	return "%!Weekday(" + string(buf[n:]) + ")"
330  }
331
332  // Computations on time.
333  //
334  // The zero value for a Time is defined to be
335  //	January 1, year 1, 00:00:00.000000000 UTC
336  // which (1) looks like a zero, or as close as you can get in a date
337  // (1-1-1 00:00:00 UTC), (2) is unlikely enough to arise in practice to
338  // be a suitable "not set" sentinel, unlike Jan 1 1970, and (3) has a
339  // non-negative year even in time zones west of UTC, unlike 1-1-0
340  // 00:00:00 UTC, which would be 12-31-(-1) 19:00:00 in New York.
341  //
342  // The zero Time value does not force a specific epoch for the time
343  // representation. For example, to use the Unix epoch internally, we
344  // could define that to distinguish a zero value from Jan 1 1970, that
345  // time would be represented by sec=-1, nsec=1e9. However, it does
346  // suggest a representation, namely using 1-1-1 00:00:00 UTC as the
347  // epoch, and that's what we do.
348  //
349  // The Add and Sub computations are oblivious to the choice of epoch.
350  //
351  // The presentation computations - year, month, minute, and so on - all
352  // rely heavily on division and modulus by positive constants. For
353  // calendrical calculations we want these divisions to round down, even
354  // for negative values, so that the remainder is always positive, but
355  // Go's division (like most hardware division instructions) rounds to
356  // zero. We can still do those computations and then adjust the result
357  // for a negative numerator, but it's annoying to write the adjustment
358  // over and over. Instead, we can change to a different epoch so long
359  // ago that all the times we care about will be positive, and then round
360  // to zero and round down coincide. These presentation routines already
361  // have to add the zone offset, so adding the translation to the
362  // alternate epoch is cheap. For example, having a non-negative time t
363  // means that we can write
364  //
365  //	sec = t % 60
366  //
368  //
369  //	sec = t % 60
370  //	if sec < 0 {
371  //		sec += 60
372  //	}
373  //
374  // everywhere.
375  //
376  // The calendar runs on an exact 400 year cycle: a 400-year calendar
377  // printed for 1970-2369 will apply as well to 2370-2769. Even the days
378  // of the week match up. It simplifies the computations to choose the
379  // cycle boundaries so that the exceptional years are always delayed as
380  // long as possible. That means choosing a year equal to 1 mod 400, so
381  // that the first leap year is the 4th year, the first missed leap year
382  // is the 100th year, and the missed missed leap year is the 400th year.
383  // So we'd prefer instead to print a calendar for 2001-2400 and reuse it
384  // for 2401-2800.
385  //
386  // Finally, it's convenient if the delta between the Unix epoch and
387  // long-ago epoch is representable by an int64 constant.
388  //
389  // These three considerations—choose an epoch as early as possible, that
390  // uses a year equal to 1 mod 400, and that is no more than 2⁶³ seconds
391  // earlier than 1970—bring us to the year -292277022399. We refer to
392  // this year as the absolute zero year, and to times measured as a uint64
393  // seconds since this year as absolute times.
394  //
395  // Times measured as an int64 seconds since the year 1—the representation
396  // used for Time's sec field—are called internal times.
397  //
398  // Times measured as an int64 seconds since the year 1970 are called Unix
399  // times.
400  //
401  // It is tempting to just use the year 1 as the absolute epoch, defining
402  // that the routines are only valid for years >= 1. However, the
403  // routines would then be invalid when displaying the epoch in time zones
404  // west of UTC, since it is year 0. It doesn't seem tenable to say that
405  // printing the zero time correctly isn't supported in half the time
406  // zones. By comparison, it's reasonable to mishandle some times in
407  // the year -292277022399.
408  //
409  // All this is opaque to clients of the API and can be changed if a
410  // better implementation presents itself.
411
412  const (
413  	// The unsigned zero year for internal calculations.
414  	// Must be 1 mod 400, and times before it will not compute correctly,
415  	// but otherwise can be changed at will.
416  	absoluteZeroYear = -292277022399
417
418  	// The year of the zero Time.
419  	// Assumed by the unixToInternal computation below.
420  	internalYear = 1
421
422  	// Offsets to convert between internal and absolute or Unix times.
423  	absoluteToInternal int64 = (absoluteZeroYear - internalYear) * 365.2425 * secondsPerDay
424  	internalToAbsolute       = -absoluteToInternal
425
426  	unixToInternal int64 = (1969*365 + 1969/4 - 1969/100 + 1969/400) * secondsPerDay
427  	internalToUnix int64 = -unixToInternal
428
429  	wallToInternal int64 = (1884*365 + 1884/4 - 1884/100 + 1884/400) * secondsPerDay
430  )
431
432  // IsZero reports whether t represents the zero time instant,
433  // January 1, year 1, 00:00:00 UTC.
434  func (t Time) IsZero() bool {
435  	return t.sec() == 0 && t.nsec() == 0
436  }
437
438  // abs returns the time t as an absolute time, adjusted by the zone offset.
439  // It is called when computing a presentation property like Month or Hour.
440  func (t Time) abs() uint64 {
441  	l := t.loc
442  	// Avoid function calls when possible.
443  	if l == nil || l == &localLoc {
444  		l = l.get()
445  	}
446  	sec := t.unixSec()
447  	if l != &utcLoc {
448  		if l.cacheZone != nil && l.cacheStart <= sec && sec < l.cacheEnd {
449  			sec += int64(l.cacheZone.offset)
450  		} else {
451  			_, offset, _, _, _ := l.lookup(sec)
452  			sec += int64(offset)
453  		}
454  	}
455  	return uint64(sec + (unixToInternal + internalToAbsolute))
456  }
457
458  // locabs is a combination of the Zone and abs methods,
459  // extracting both return values from a single zone lookup.
460  func (t Time) locabs() (name string, offset int, abs uint64) {
461  	l := t.loc
462  	if l == nil || l == &localLoc {
463  		l = l.get()
464  	}
465  	// Avoid function call if we hit the local time cache.
466  	sec := t.unixSec()
467  	if l != &utcLoc {
468  		if l.cacheZone != nil && l.cacheStart <= sec && sec < l.cacheEnd {
469  			name = l.cacheZone.name
470  			offset = l.cacheZone.offset
471  		} else {
472  			name, offset, _, _, _ = l.lookup(sec)
473  		}
474  		sec += int64(offset)
475  	} else {
476  		name = "UTC"
477  	}
478  	abs = uint64(sec + (unixToInternal + internalToAbsolute))
479  	return
480  }
481
482  // Date returns the year, month, and day in which t occurs.
483  func (t Time) Date() (year int, month Month, day int) {
484  	year, month, day, _ = t.date(true)
485  	return
486  }
487
488  // Year returns the year in which t occurs.
489  func (t Time) Year() int {
490  	year, _, _, _ := t.date(false)
491  	return year
492  }
493
494  // Month returns the month of the year specified by t.
495  func (t Time) Month() Month {
496  	_, month, _, _ := t.date(true)
497  	return month
498  }
499
500  // Day returns the day of the month specified by t.
501  func (t Time) Day() int {
502  	_, _, day, _ := t.date(true)
503  	return day
504  }
505
506  // Weekday returns the day of the week specified by t.
507  func (t Time) Weekday() Weekday {
508  	return absWeekday(t.abs())
509  }
510
511  // absWeekday is like Weekday but operates on an absolute time.
512  func absWeekday(abs uint64) Weekday {
513  	// January 1 of the absolute year, like January 1 of 2001, was a Monday.
514  	sec := (abs + uint64(Monday)*secondsPerDay) % secondsPerWeek
515  	return Weekday(int(sec) / secondsPerDay)
516  }
517
518  // ISOWeek returns the ISO 8601 year and week number in which t occurs.
519  // Week ranges from 1 to 53. Jan 01 to Jan 03 of year n might belong to
520  // week 52 or 53 of year n-1, and Dec 29 to Dec 31 might belong to week 1
521  // of year n+1.
522  func (t Time) ISOWeek() (year, week int) {
523  	// According to the rule that the first calendar week of a calendar year is
524  	// the week including the first Thursday of that year, and that the last one is
525  	// the week immediately preceding the first calendar week of the next calendar year.
526  	// See https://www.iso.org/obp/ui#iso:std:iso:8601:-1:ed-1:v1:en:term:3.1.1.23 for details.
527
529  	// Monday Tuesday Wednesday Thursday Friday Saturday Sunday
530  	// 1      2       3         4        5      6        7
531  	// +3     +2      +1        0        -1     -2       -3
532  	// the offset to Thursday
533  	abs := t.abs()
534  	d := Thursday - absWeekday(abs)
535  	// handle Sunday
536  	if d == 4 {
537  		d = -3
538  	}
539  	// find the Thursday of the calendar week
540  	abs += uint64(d) * secondsPerDay
541  	year, _, _, yday := absDate(abs, false)
542  	return year, yday/7 + 1
543  }
544
545  // Clock returns the hour, minute, and second within the day specified by t.
546  func (t Time) Clock() (hour, min, sec int) {
547  	return absClock(t.abs())
548  }
549
550  // absClock is like clock but operates on an absolute time.
551  func absClock(abs uint64) (hour, min, sec int) {
552  	sec = int(abs % secondsPerDay)
553  	hour = sec / secondsPerHour
554  	sec -= hour * secondsPerHour
555  	min = sec / secondsPerMinute
556  	sec -= min * secondsPerMinute
557  	return
558  }
559
560  // Hour returns the hour within the day specified by t, in the range [0, 23].
561  func (t Time) Hour() int {
562  	return int(t.abs()%secondsPerDay) / secondsPerHour
563  }
564
565  // Minute returns the minute offset within the hour specified by t, in the range [0, 59].
566  func (t Time) Minute() int {
567  	return int(t.abs()%secondsPerHour) / secondsPerMinute
568  }
569
570  // Second returns the second offset within the minute specified by t, in the range [0, 59].
571  func (t Time) Second() int {
572  	return int(t.abs() % secondsPerMinute)
573  }
574
575  // Nanosecond returns the nanosecond offset within the second specified by t,
576  // in the range [0, 999999999].
577  func (t Time) Nanosecond() int {
578  	return int(t.nsec())
579  }
580
581  // YearDay returns the day of the year specified by t, in the range [1,365] for non-leap years,
582  // and [1,366] in leap years.
583  func (t Time) YearDay() int {
584  	_, _, _, yday := t.date(false)
585  	return yday + 1
586  }
587
588  // A Duration represents the elapsed time between two instants
589  // as an int64 nanosecond count. The representation limits the
590  // largest representable duration to approximately 290 years.
591  type Duration int64
592
593  const (
594  	minDuration Duration = -1 << 63
595  	maxDuration Duration = 1<<63 - 1
596  )
597
598  // Common durations. There is no definition for units of Day or larger
599  // to avoid confusion across daylight savings time zone transitions.
600  //
601  // To count the number of units in a Duration, divide:
602  //
603  //	second := time.Second
604  //	fmt.Print(int64(second/time.Millisecond)) // prints 1000
605  //
606  // To convert an integer number of units to a Duration, multiply:
607  //
608  //	seconds := 10
609  //	fmt.Print(time.Duration(seconds)*time.Second) // prints 10s
610  const (
611  	Nanosecond  Duration = 1
612  	Microsecond          = 1000 * Nanosecond
613  	Millisecond          = 1000 * Microsecond
614  	Second               = 1000 * Millisecond
615  	Minute               = 60 * Second
616  	Hour                 = 60 * Minute
617  )
618
619  // String returns a string representing the duration in the form "72h3m0.5s".
620  // Leading zero units are omitted. As a special case, durations less than one
621  // second format use a smaller unit (milli-, micro-, or nanoseconds) to ensure
622  // that the leading digit is non-zero. The zero duration formats as 0s.
623  func (d Duration) String() string {
624  	// Largest time is 2540400h10m10.000000000s
625  	var buf [32]byte
626  	w := len(buf)
627
628  	u := uint64(d)
629  	neg := d < 0
630  	if neg {
631  		u = -u
632  	}
633
634  	if u < uint64(Second) {
635  		// Special case: if duration is smaller than a second,
636  		// use smaller units, like 1.2ms
637  		var prec int
638  		w--
639  		buf[w] = 's'
640  		w--
641  		switch {
642  		case u == 0:
643  			return "0s"
644  		case u < uint64(Microsecond):
645  			// print nanoseconds
646  			prec = 0
647  			buf[w] = 'n'
648  		case u < uint64(Millisecond):
649  			// print microseconds
650  			prec = 3
651  			// U+00B5 'µ' micro sign == 0xC2 0xB5
652  			w-- // Need room for two bytes.
653  			copy(buf[w:], "µ")
654  		default:
655  			// print milliseconds
656  			prec = 6
657  			buf[w] = 'm'
658  		}
659  		w, u = fmtFrac(buf[:w], u, prec)
660  		w = fmtInt(buf[:w], u)
661  	} else {
662  		w--
663  		buf[w] = 's'
664
665  		w, u = fmtFrac(buf[:w], u, 9)
666
667  		// u is now integer seconds
668  		w = fmtInt(buf[:w], u%60)
669  		u /= 60
670
671  		// u is now integer minutes
672  		if u > 0 {
673  			w--
674  			buf[w] = 'm'
675  			w = fmtInt(buf[:w], u%60)
676  			u /= 60
677
678  			// u is now integer hours
679  			// Stop at hours because days can be different lengths.
680  			if u > 0 {
681  				w--
682  				buf[w] = 'h'
683  				w = fmtInt(buf[:w], u)
684  			}
685  		}
686  	}
687
688  	if neg {
689  		w--
690  		buf[w] = '-'
691  	}
692
693  	return string(buf[w:])
694  }
695
696  // fmtFrac formats the fraction of v/10**prec (e.g., ".12345") into the
697  // tail of buf, omitting trailing zeros. It omits the decimal
698  // point too when the fraction is 0. It returns the index where the
699  // output bytes begin and the value v/10**prec.
700  func fmtFrac(buf []byte, v uint64, prec int) (nw int, nv uint64) {
701  	// Omit trailing zeros up to and including decimal point.
702  	w := len(buf)
703  	print := false
704  	for i := 0; i < prec; i++ {
705  		digit := v % 10
706  		print = print || digit != 0
707  		if print {
708  			w--
709  			buf[w] = byte(digit) + '0'
710  		}
711  		v /= 10
712  	}
713  	if print {
714  		w--
715  		buf[w] = '.'
716  	}
717  	return w, v
718  }
719
720  // fmtInt formats v into the tail of buf.
721  // It returns the index where the output begins.
722  func fmtInt(buf []byte, v uint64) int {
723  	w := len(buf)
724  	if v == 0 {
725  		w--
726  		buf[w] = '0'
727  	} else {
728  		for v > 0 {
729  			w--
730  			buf[w] = byte(v%10) + '0'
731  			v /= 10
732  		}
733  	}
734  	return w
735  }
736
737  // Nanoseconds returns the duration as an integer nanosecond count.
738  func (d Duration) Nanoseconds() int64 { return int64(d) }
739
740  // Microseconds returns the duration as an integer microsecond count.
741  func (d Duration) Microseconds() int64 { return int64(d) / 1e3 }
742
743  // Milliseconds returns the duration as an integer millisecond count.
744  func (d Duration) Milliseconds() int64 { return int64(d) / 1e6 }
745
746  // These methods return float64 because the dominant
747  // use case is for printing a floating point number like 1.5s, and
748  // a truncation to integer would make them not useful in those cases.
749  // Splitting the integer and fraction ourselves guarantees that
750  // converting the returned float64 to an integer rounds the same
751  // way that a pure integer conversion would have, even in cases
752  // where, say, float64(d.Nanoseconds())/1e9 would have rounded
753  // differently.
754
755  // Seconds returns the duration as a floating point number of seconds.
756  func (d Duration) Seconds() float64 {
757  	sec := d / Second
758  	nsec := d % Second
759  	return float64(sec) + float64(nsec)/1e9
760  }
761
762  // Minutes returns the duration as a floating point number of minutes.
763  func (d Duration) Minutes() float64 {
764  	min := d / Minute
765  	nsec := d % Minute
766  	return float64(min) + float64(nsec)/(60*1e9)
767  }
768
769  // Hours returns the duration as a floating point number of hours.
770  func (d Duration) Hours() float64 {
771  	hour := d / Hour
772  	nsec := d % Hour
773  	return float64(hour) + float64(nsec)/(60*60*1e9)
774  }
775
776  // Truncate returns the result of rounding d toward zero to a multiple of m.
777  // If m <= 0, Truncate returns d unchanged.
778  func (d Duration) Truncate(m Duration) Duration {
779  	if m <= 0 {
780  		return d
781  	}
782  	return d - d%m
783  }
784
785  // lessThanHalf reports whether x+x < y but avoids overflow,
786  // assuming x and y are both positive (Duration is signed).
787  func lessThanHalf(x, y Duration) bool {
788  	return uint64(x)+uint64(x) < uint64(y)
789  }
790
791  // Round returns the result of rounding d to the nearest multiple of m.
792  // The rounding behavior for halfway values is to round away from zero.
793  // If the result exceeds the maximum (or minimum)
794  // value that can be stored in a Duration,
795  // Round returns the maximum (or minimum) duration.
796  // If m <= 0, Round returns d unchanged.
797  func (d Duration) Round(m Duration) Duration {
798  	if m <= 0 {
799  		return d
800  	}
801  	r := d % m
802  	if d < 0 {
803  		r = -r
804  		if lessThanHalf(r, m) {
805  			return d + r
806  		}
807  		if d1 := d - m + r; d1 < d {
808  			return d1
809  		}
810  		return minDuration // overflow
811  	}
812  	if lessThanHalf(r, m) {
813  		return d - r
814  	}
815  	if d1 := d + m - r; d1 > d {
816  		return d1
817  	}
818  	return maxDuration // overflow
819  }
820
821  // Abs returns the absolute value of d.
822  // As a special case, math.MinInt64 is converted to math.MaxInt64.
823  func (d Duration) Abs() Duration {
824  	switch {
825  	case d >= 0:
826  		return d
827  	case d == minDuration:
828  		return maxDuration
829  	default:
830  		return -d
831  	}
832  }
833
834  // Add returns the time t+d.
835  func (t Time) Add(d Duration) Time {
836  	dsec := int64(d / 1e9)
837  	nsec := t.nsec() + int32(d%1e9)
838  	if nsec >= 1e9 {
839  		dsec++
840  		nsec -= 1e9
841  	} else if nsec < 0 {
842  		dsec--
843  		nsec += 1e9
844  	}
845  	t.wall = t.wall&^nsecMask | uint64(nsec) // update nsec
847  	if t.wall&hasMonotonic != 0 {
848  		te := t.ext + int64(d)
849  		if d < 0 && te > t.ext || d > 0 && te < t.ext {
850  			// Monotonic clock reading now out of range; degrade to wall-only.
851  			t.stripMono()
852  		} else {
853  			t.ext = te
854  		}
855  	}
856  	return t
857  }
858
859  // Sub returns the duration t-u. If the result exceeds the maximum (or minimum)
860  // value that can be stored in a Duration, the maximum (or minimum) duration
861  // will be returned.
862  // To compute t-d for a duration d, use t.Add(-d).
863  func (t Time) Sub(u Time) Duration {
864  	if t.wall&u.wall&hasMonotonic != 0 {
865  		te := t.ext
866  		ue := u.ext
867  		d := Duration(te - ue)
868  		if d < 0 && te > ue {
869  			return maxDuration // t - u is positive out of range
870  		}
871  		if d > 0 && te < ue {
872  			return minDuration // t - u is negative out of range
873  		}
874  		return d
875  	}
876  	d := Duration(t.sec()-u.sec())*Second + Duration(t.nsec()-u.nsec())
877  	// Check for overflow or underflow.
878  	switch {
880  		return d // d is correct
881  	case t.Before(u):
882  		return minDuration // t - u is negative out of range
883  	default:
884  		return maxDuration // t - u is positive out of range
885  	}
886  }
887
888  // Since returns the time elapsed since t.
889  // It is shorthand for time.Now().Sub(t).
890  func Since(t Time) Duration {
891  	var now Time
892  	if t.wall&hasMonotonic != 0 {
893  		// Common case optimization: if t has monotonic time, then Sub will use only it.
894  		now = Time{hasMonotonic, runtimeNano() - startNano, nil}
895  	} else {
896  		now = Now()
897  	}
898  	return now.Sub(t)
899  }
900
901  // Until returns the duration until t.
902  // It is shorthand for t.Sub(time.Now()).
903  func Until(t Time) Duration {
904  	var now Time
905  	if t.wall&hasMonotonic != 0 {
906  		// Common case optimization: if t has monotonic time, then Sub will use only it.
907  		now = Time{hasMonotonic, runtimeNano() - startNano, nil}
908  	} else {
909  		now = Now()
910  	}
911  	return t.Sub(now)
912  }
913
915  // given number of years, months, and days to t.
916  // For example, AddDate(-1, 2, 3) applied to January 1, 2011
917  // returns March 4, 2010.
918  //
919  // AddDate normalizes its result in the same way that Date does,
920  // so, for example, adding one month to October 31 yields
921  // December 1, the normalized form for November 31.
922  func (t Time) AddDate(years int, months int, days int) Time {
923  	year, month, day := t.Date()
924  	hour, min, sec := t.Clock()
925  	return Date(year+years, month+Month(months), day+days, hour, min, sec, int(t.nsec()), t.Location())
926  }
927
928  const (
929  	secondsPerMinute = 60
930  	secondsPerHour   = 60 * secondsPerMinute
931  	secondsPerDay    = 24 * secondsPerHour
932  	secondsPerWeek   = 7 * secondsPerDay
933  	daysPer400Years  = 365*400 + 97
934  	daysPer100Years  = 365*100 + 24
935  	daysPer4Years    = 365*4 + 1
936  )
937
938  // date computes the year, day of year, and when full=true,
939  // the month and day in which t occurs.
940  func (t Time) date(full bool) (year int, month Month, day int, yday int) {
941  	return absDate(t.abs(), full)
942  }
943
944  // absDate is like date but operates on an absolute time.
945  func absDate(abs uint64, full bool) (year int, month Month, day int, yday int) {
946  	// Split into time and day.
947  	d := abs / secondsPerDay
948
949  	// Account for 400 year cycles.
950  	n := d / daysPer400Years
951  	y := 400 * n
952  	d -= daysPer400Years * n
953
954  	// Cut off 100-year cycles.
955  	// The last cycle has one extra leap year, so on the last day
956  	// of that year, day / daysPer100Years will be 4 instead of 3.
957  	// Cut it back down to 3 by subtracting n>>2.
958  	n = d / daysPer100Years
959  	n -= n >> 2
960  	y += 100 * n
961  	d -= daysPer100Years * n
962
963  	// Cut off 4-year cycles.
964  	// The last cycle has a missing leap year, which does not
965  	// affect the computation.
966  	n = d / daysPer4Years
967  	y += 4 * n
968  	d -= daysPer4Years * n
969
970  	// Cut off years within a 4-year cycle.
971  	// The last year is a leap year, so on the last day of that year,
972  	// day / 365 will be 4 instead of 3. Cut it back down to 3
973  	// by subtracting n>>2.
974  	n = d / 365
975  	n -= n >> 2
976  	y += n
977  	d -= 365 * n
978
979  	year = int(int64(y) + absoluteZeroYear)
980  	yday = int(d)
981
982  	if !full {
983  		return
984  	}
985
986  	day = yday
987  	if isLeap(year) {
988  		// Leap year
989  		switch {
990  		case day > 31+29-1:
991  			// After leap day; pretend it wasn't there.
992  			day--
993  		case day == 31+29-1:
994  			// Leap day.
995  			month = February
996  			day = 29
997  			return
998  		}
999  	}
1000
1001  	// Estimate month on assumption that every month has 31 days.
1002  	// The estimate may be too low by at most one month, so adjust.
1003  	month = Month(day / 31)
1004  	end := int(daysBefore[month+1])
1005  	var begin int
1006  	if day >= end {
1007  		month++
1008  		begin = end
1009  	} else {
1010  		begin = int(daysBefore[month])
1011  	}
1012
1013  	month++ // because January is 1
1014  	day = day - begin + 1
1015  	return
1016  }
1017
1018  // daysBefore[m] counts the number of days in a non-leap year
1019  // before month m begins. There is an entry for m=12, counting
1020  // the number of days before January of next year (365).
1021  var daysBefore = [...]int32{
1022  	0,
1023  	31,
1024  	31 + 28,
1025  	31 + 28 + 31,
1026  	31 + 28 + 31 + 30,
1027  	31 + 28 + 31 + 30 + 31,
1028  	31 + 28 + 31 + 30 + 31 + 30,
1029  	31 + 28 + 31 + 30 + 31 + 30 + 31,
1030  	31 + 28 + 31 + 30 + 31 + 30 + 31 + 31,
1031  	31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30,
1032  	31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31,
1033  	31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31 + 30,
1034  	31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31 + 30 + 31,
1035  }
1036
1037  func daysIn(m Month, year int) int {
1038  	if m == February && isLeap(year) {
1039  		return 29
1040  	}
1041  	return int(daysBefore[m] - daysBefore[m-1])
1042  }
1043
1044  // daysSinceEpoch takes a year and returns the number of days from
1045  // the absolute epoch to the start of that year.
1046  // This is basically (year - zeroYear) * 365, but accounting for leap days.
1047  func daysSinceEpoch(year int) uint64 {
1048  	y := uint64(int64(year) - absoluteZeroYear)
1049
1050  	// Add in days from 400-year cycles.
1051  	n := y / 400
1052  	y -= 400 * n
1053  	d := daysPer400Years * n
1054
1055  	// Add in 100-year cycles.
1056  	n = y / 100
1057  	y -= 100 * n
1058  	d += daysPer100Years * n
1059
1060  	// Add in 4-year cycles.
1061  	n = y / 4
1062  	y -= 4 * n
1063  	d += daysPer4Years * n
1064
1065  	// Add in non-leap years.
1066  	n = y
1067  	d += 365 * n
1068
1069  	return d
1070  }
1071
1072  // Provided by package runtime.
1073  func now() (sec int64, nsec int32, mono int64)
1074
1075  // runtimeNano returns the current value of the runtime clock in nanoseconds.
1076  //
1078  func runtimeNano() int64
1079
1080  // Monotonic times are reported as offsets from startNano.
1081  // We initialize startNano to runtimeNano() - 1 so that on systems where
1082  // monotonic time resolution is fairly low (e.g. Windows 2008
1083  // which appears to have a default resolution of 15ms),
1084  // we avoid ever reporting a monotonic time of 0.
1085  // (Callers may want to use 0 as "time not set".)
1086  var startNano int64 = runtimeNano() - 1
1087
1088  // Now returns the current local time.
1089  func Now() Time {
1090  	sec, nsec, mono := now()
1091  	mono -= startNano
1092  	sec += unixToInternal - minWall
1093  	if uint64(sec)>>33 != 0 {
1094  		return Time{uint64(nsec), sec + minWall, Local}
1095  	}
1096  	return Time{hasMonotonic | uint64(sec)<<nsecShift | uint64(nsec), mono, Local}
1097  }
1098
1099  func unixTime(sec int64, nsec int32) Time {
1100  	return Time{uint64(nsec), sec + unixToInternal, Local}
1101  }
1102
1103  // UTC returns t with the location set to UTC.
1104  func (t Time) UTC() Time {
1105  	t.setLoc(&utcLoc)
1106  	return t
1107  }
1108
1109  // Local returns t with the location set to local time.
1110  func (t Time) Local() Time {
1111  	t.setLoc(Local)
1112  	return t
1113  }
1114
1115  // In returns a copy of t representing the same time instant, but
1116  // with the copy's location information set to loc for display
1117  // purposes.
1118  //
1119  // In panics if loc is nil.
1120  func (t Time) In(loc *Location) Time {
1121  	if loc == nil {
1122  		panic("time: missing Location in call to Time.In")
1123  	}
1124  	t.setLoc(loc)
1125  	return t
1126  }
1127
1128  // Location returns the time zone information associated with t.
1129  func (t Time) Location() *Location {
1130  	l := t.loc
1131  	if l == nil {
1132  		l = UTC
1133  	}
1134  	return l
1135  }
1136
1137  // Zone computes the time zone in effect at time t, returning the abbreviated
1138  // name of the zone (such as "CET") and its offset in seconds east of UTC.
1139  func (t Time) Zone() (name string, offset int) {
1140  	name, offset, _, _, _ = t.loc.lookup(t.unixSec())
1141  	return
1142  }
1143
1144  // ZoneBounds returns the bounds of the time zone in effect at time t.
1145  // The zone begins at start and the next zone begins at end.
1146  // If the zone begins at the beginning of time, start will be returned as a zero Time.
1147  // If the zone goes on forever, end will be returned as a zero Time.
1148  // The Location of the returned times will be the same as t.
1149  func (t Time) ZoneBounds() (start, end Time) {
1150  	_, _, startSec, endSec, _ := t.loc.lookup(t.unixSec())
1151  	if startSec != alpha {
1152  		start = unixTime(startSec, 0)
1153  		start.setLoc(t.loc)
1154  	}
1155  	if endSec != omega {
1156  		end = unixTime(endSec, 0)
1157  		end.setLoc(t.loc)
1158  	}
1159  	return
1160  }
1161
1162  // Unix returns t as a Unix time, the number of seconds elapsed
1163  // since January 1, 1970 UTC. The result does not depend on the
1164  // location associated with t.
1165  // Unix-like operating systems often record time as a 32-bit
1166  // count of seconds, but since the method here returns a 64-bit
1167  // value it is valid for billions of years into the past or future.
1168  func (t Time) Unix() int64 {
1169  	return t.unixSec()
1170  }
1171
1172  // UnixMilli returns t as a Unix time, the number of milliseconds elapsed since
1173  // January 1, 1970 UTC. The result is undefined if the Unix time in
1174  // milliseconds cannot be represented by an int64 (a date more than 292 million
1175  // years before or after 1970). The result does not depend on the
1176  // location associated with t.
1177  func (t Time) UnixMilli() int64 {
1178  	return t.unixSec()*1e3 + int64(t.nsec())/1e6
1179  }
1180
1181  // UnixMicro returns t as a Unix time, the number of microseconds elapsed since
1182  // January 1, 1970 UTC. The result is undefined if the Unix time in
1183  // microseconds cannot be represented by an int64 (a date before year -290307 or
1184  // after year 294246). The result does not depend on the location associated
1185  // with t.
1186  func (t Time) UnixMicro() int64 {
1187  	return t.unixSec()*1e6 + int64(t.nsec())/1e3
1188  }
1189
1190  // UnixNano returns t as a Unix time, the number of nanoseconds elapsed
1191  // since January 1, 1970 UTC. The result is undefined if the Unix time
1192  // in nanoseconds cannot be represented by an int64 (a date before the year
1193  // 1678 or after 2262). Note that this means the result of calling UnixNano
1194  // on the zero Time is undefined. The result does not depend on the
1195  // location associated with t.
1196  func (t Time) UnixNano() int64 {
1197  	return (t.unixSec())*1e9 + int64(t.nsec())
1198  }
1199
1200  const (
1201  	timeBinaryVersionV1 byte = iota + 1 // For general situation
1202  	timeBinaryVersionV2                 // For LMT only
1203  )
1204
1205  // MarshalBinary implements the encoding.BinaryMarshaler interface.
1206  func (t Time) MarshalBinary() ([]byte, error) {
1207  	var offsetMin int16 // minutes east of UTC. -1 is UTC.
1208  	var offsetSec int8
1209  	version := timeBinaryVersionV1
1210
1211  	if t.Location() == UTC {
1212  		offsetMin = -1
1213  	} else {
1214  		_, offset := t.Zone()
1215  		if offset%60 != 0 {
1216  			version = timeBinaryVersionV2
1217  			offsetSec = int8(offset % 60)
1218  		}
1219
1220  		offset /= 60
1221  		if offset < -32768 || offset == -1 || offset > 32767 {
1222  			return nil, errors.New("Time.MarshalBinary: unexpected zone offset")
1223  		}
1224  		offsetMin = int16(offset)
1225  	}
1226
1227  	sec := t.sec()
1228  	nsec := t.nsec()
1229  	enc := []byte{
1230  		version,         // byte 0 : version
1231  		byte(sec >> 56), // bytes 1-8: seconds
1232  		byte(sec >> 48),
1233  		byte(sec >> 40),
1234  		byte(sec >> 32),
1235  		byte(sec >> 24),
1236  		byte(sec >> 16),
1237  		byte(sec >> 8),
1238  		byte(sec),
1239  		byte(nsec >> 24), // bytes 9-12: nanoseconds
1240  		byte(nsec >> 16),
1241  		byte(nsec >> 8),
1242  		byte(nsec),
1243  		byte(offsetMin >> 8), // bytes 13-14: zone offset in minutes
1244  		byte(offsetMin),
1245  	}
1246  	if version == timeBinaryVersionV2 {
1247  		enc = append(enc, byte(offsetSec))
1248  	}
1249
1250  	return enc, nil
1251  }
1252
1253  // UnmarshalBinary implements the encoding.BinaryUnmarshaler interface.
1254  func (t *Time) UnmarshalBinary(data []byte) error {
1255  	buf := data
1256  	if len(buf) == 0 {
1257  		return errors.New("Time.UnmarshalBinary: no data")
1258  	}
1259
1260  	version := buf[0]
1261  	if version != timeBinaryVersionV1 && version != timeBinaryVersionV2 {
1262  		return errors.New("Time.UnmarshalBinary: unsupported version")
1263  	}
1264
1265  	wantLen := /*version*/ 1 + /*sec*/ 8 + /*nsec*/ 4 + /*zone offset*/ 2
1266  	if version == timeBinaryVersionV2 {
1267  		wantLen++
1268  	}
1269  	if len(buf) != wantLen {
1270  		return errors.New("Time.UnmarshalBinary: invalid length")
1271  	}
1272
1273  	buf = buf[1:]
1274  	sec := int64(buf[7]) | int64(buf[6])<<8 | int64(buf[5])<<16 | int64(buf[4])<<24 |
1275  		int64(buf[3])<<32 | int64(buf[2])<<40 | int64(buf[1])<<48 | int64(buf[0])<<56
1276
1277  	buf = buf[8:]
1278  	nsec := int32(buf[3]) | int32(buf[2])<<8 | int32(buf[1])<<16 | int32(buf[0])<<24
1279
1280  	buf = buf[4:]
1281  	offset := int(int16(buf[1])|int16(buf[0])<<8) * 60
1282  	if version == timeBinaryVersionV2 {
1283  		offset += int(buf[2])
1284  	}
1285
1286  	*t = Time{}
1287  	t.wall = uint64(nsec)
1288  	t.ext = sec
1289
1290  	if offset == -1*60 {
1291  		t.setLoc(&utcLoc)
1292  	} else if _, localoff, _, _, _ := Local.lookup(t.unixSec()); offset == localoff {
1293  		t.setLoc(Local)
1294  	} else {
1295  		t.setLoc(FixedZone("", offset))
1296  	}
1297
1298  	return nil
1299  }
1300
1301  // TODO(rsc): Remove GobEncoder, GobDecoder, MarshalJSON, UnmarshalJSON in Go 2.
1302  // The same semantics will be provided by the generic MarshalBinary, MarshalText,
1303  // UnmarshalBinary, UnmarshalText.
1304
1305  // GobEncode implements the gob.GobEncoder interface.
1306  func (t Time) GobEncode() ([]byte, error) {
1307  	return t.MarshalBinary()
1308  }
1309
1310  // GobDecode implements the gob.GobDecoder interface.
1311  func (t *Time) GobDecode(data []byte) error {
1312  	return t.UnmarshalBinary(data)
1313  }
1314
1315  // MarshalJSON implements the json.Marshaler interface.
1316  // The time is a quoted string in RFC 3339 format, with sub-second precision added if present.
1317  func (t Time) MarshalJSON() ([]byte, error) {
1318  	if y := t.Year(); y < 0 || y >= 10000 {
1319  		// RFC 3339 is clear that years are 4 digits exactly.
1320  		// See golang.org/issue/4556#c15 for more discussion.
1321  		return nil, errors.New("Time.MarshalJSON: year outside of range [0,9999]")
1322  	}
1323
1324  	b := make([]byte, 0, len(RFC3339Nano)+2)
1325  	b = append(b, '"')
1326  	b = t.AppendFormat(b, RFC3339Nano)
1327  	b = append(b, '"')
1328  	return b, nil
1329  }
1330
1331  // UnmarshalJSON implements the json.Unmarshaler interface.
1332  // The time is expected to be a quoted string in RFC 3339 format.
1333  func (t *Time) UnmarshalJSON(data []byte) error {
1334  	// Ignore null, like in the main JSON package.
1335  	if string(data) == "null" {
1336  		return nil
1337  	}
1338  	// Fractional seconds are handled implicitly by Parse.
1339  	var err error
1340  	*t, err = Parse(`"`+RFC3339+`"`, string(data))
1341  	return err
1342  }
1343
1344  // MarshalText implements the encoding.TextMarshaler interface.
1345  // The time is formatted in RFC 3339 format, with sub-second precision added if present.
1346  func (t Time) MarshalText() ([]byte, error) {
1347  	if y := t.Year(); y < 0 || y >= 10000 {
1348  		return nil, errors.New("Time.MarshalText: year outside of range [0,9999]")
1349  	}
1350
1351  	b := make([]byte, 0, len(RFC3339Nano))
1352  	return t.AppendFormat(b, RFC3339Nano), nil
1353  }
1354
1355  // UnmarshalText implements the encoding.TextUnmarshaler interface.
1356  // The time is expected to be in RFC 3339 format.
1357  func (t *Time) UnmarshalText(data []byte) error {
1358  	// Fractional seconds are handled implicitly by Parse.
1359  	var err error
1360  	*t, err = Parse(RFC3339, string(data))
1361  	return err
1362  }
1363
1364  // Unix returns the local Time corresponding to the given Unix time,
1365  // sec seconds and nsec nanoseconds since January 1, 1970 UTC.
1366  // It is valid to pass nsec outside the range [0, 999999999].
1367  // Not all sec values have a corresponding time value. One such
1368  // value is 1<<63-1 (the largest int64 value).
1369  func Unix(sec int64, nsec int64) Time {
1370  	if nsec < 0 || nsec >= 1e9 {
1371  		n := nsec / 1e9
1372  		sec += n
1373  		nsec -= n * 1e9
1374  		if nsec < 0 {
1375  			nsec += 1e9
1376  			sec--
1377  		}
1378  	}
1379  	return unixTime(sec, int32(nsec))
1380  }
1381
1382  // UnixMilli returns the local Time corresponding to the given Unix time,
1383  // msec milliseconds since January 1, 1970 UTC.
1384  func UnixMilli(msec int64) Time {
1385  	return Unix(msec/1e3, (msec%1e3)*1e6)
1386  }
1387
1388  // UnixMicro returns the local Time corresponding to the given Unix time,
1389  // usec microseconds since January 1, 1970 UTC.
1390  func UnixMicro(usec int64) Time {
1391  	return Unix(usec/1e6, (usec%1e6)*1e3)
1392  }
1393
1394  // IsDST reports whether the time in the configured location is in Daylight Savings Time.
1395  func (t Time) IsDST() bool {
1396  	_, _, _, _, isDST := t.loc.lookup(t.Unix())
1397  	return isDST
1398  }
1399
1400  func isLeap(year int) bool {
1401  	return year%4 == 0 && (year%100 != 0 || year%400 == 0)
1402  }
1403
1404  // norm returns nhi, nlo such that
1405  //
1406  //	hi * base + lo == nhi * base + nlo
1407  //	0 <= nlo < base
1408  func norm(hi, lo, base int) (nhi, nlo int) {
1409  	if lo < 0 {
1410  		n := (-lo-1)/base + 1
1411  		hi -= n
1412  		lo += n * base
1413  	}
1414  	if lo >= base {
1415  		n := lo / base
1416  		hi += n
1417  		lo -= n * base
1418  	}
1419  	return hi, lo
1420  }
1421
1422  // Date returns the Time corresponding to
1423  //
1424  //	yyyy-mm-dd hh:mm:ss + nsec nanoseconds
1425  //
1426  // in the appropriate zone for that time in the given location.
1427  //
1428  // The month, day, hour, min, sec, and nsec values may be outside
1429  // their usual ranges and will be normalized during the conversion.
1430  // For example, October 32 converts to November 1.
1431  //
1432  // A daylight savings time transition skips or repeats times.
1433  // For example, in the United States, March 13, 2011 2:15am never occurred,
1434  // while November 6, 2011 1:15am occurred twice. In such cases, the
1435  // choice of time zone, and therefore the time, is not well-defined.
1436  // Date returns a time that is correct in one of the two zones involved
1437  // in the transition, but it does not guarantee which.
1438  //
1439  // Date panics if loc is nil.
1440  func Date(year int, month Month, day, hour, min, sec, nsec int, loc *Location) Time {
1441  	if loc == nil {
1442  		panic("time: missing Location in call to Date")
1443  	}
1444
1445  	// Normalize month, overflowing into year.
1446  	m := int(month) - 1
1447  	year, m = norm(year, m, 12)
1448  	month = Month(m) + 1
1449
1450  	// Normalize nsec, sec, min, hour, overflowing into day.
1451  	sec, nsec = norm(sec, nsec, 1e9)
1452  	min, sec = norm(min, sec, 60)
1453  	hour, min = norm(hour, min, 60)
1454  	day, hour = norm(day, hour, 24)
1455
1456  	// Compute days since the absolute epoch.
1457  	d := daysSinceEpoch(year)
1458
1459  	// Add in days before this month.
1460  	d += uint64(daysBefore[month-1])
1461  	if isLeap(year) && month >= March {
1462  		d++ // February 29
1463  	}
1464
1465  	// Add in days before today.
1466  	d += uint64(day - 1)
1467
1468  	// Add in time elapsed today.
1469  	abs := d * secondsPerDay
1470  	abs += uint64(hour*secondsPerHour + min*secondsPerMinute + sec)
1471
1472  	unix := int64(abs) + (absoluteToInternal + internalToUnix)
1473
1474  	// Look for zone offset for expected time, so we can adjust to UTC.
1475  	// The lookup function expects UTC, so first we pass unix in the
1476  	// hope that it will not be too close to a zone transition,
1477  	// and then adjust if it is.
1478  	_, offset, start, end, _ := loc.lookup(unix)
1479  	if offset != 0 {
1480  		utc := unix - int64(offset)
1481  		// If utc is valid for the time zone we found, then we have the right offset.
1482  		// If not, we get the correct offset by looking up utc in the location.
1483  		if utc < start || utc >= end {
1484  			_, offset, _, _, _ = loc.lookup(utc)
1485  		}
1486  		unix -= int64(offset)
1487  	}
1488
1489  	t := unixTime(unix, int32(nsec))
1490  	t.setLoc(loc)
1491  	return t
1492  }
1493
1494  // Truncate returns the result of rounding t down to a multiple of d (since the zero time).
1495  // If d <= 0, Truncate returns t stripped of any monotonic clock reading but otherwise unchanged.
1496  //
1497  // Truncate operates on the time as an absolute duration since the
1498  // zero time; it does not operate on the presentation form of the
1499  // time. Thus, Truncate(Hour) may return a time with a non-zero
1500  // minute, depending on the time's Location.
1501  func (t Time) Truncate(d Duration) Time {
1502  	t.stripMono()
1503  	if d <= 0 {
1504  		return t
1505  	}
1506  	_, r := div(t, d)
1508  }
1509
1510  // Round returns the result of rounding t to the nearest multiple of d (since the zero time).
1511  // The rounding behavior for halfway values is to round up.
1512  // If d <= 0, Round returns t stripped of any monotonic clock reading but otherwise unchanged.
1513  //
1514  // Round operates on the time as an absolute duration since the
1515  // zero time; it does not operate on the presentation form of the
1516  // time. Thus, Round(Hour) may return a time with a non-zero
1517  // minute, depending on the time's Location.
1518  func (t Time) Round(d Duration) Time {
1519  	t.stripMono()
1520  	if d <= 0 {
1521  		return t
1522  	}
1523  	_, r := div(t, d)
1524  	if lessThanHalf(r, d) {
1526  	}
1528  }
1529
1530  // div divides t by d and returns the quotient parity and remainder.
1531  // We don't use the quotient parity anymore (round half up instead of round to even)
1532  // but it's still here in case we change our minds.
1533  func div(t Time, d Duration) (qmod2 int, r Duration) {
1534  	neg := false
1535  	nsec := t.nsec()
1536  	sec := t.sec()
1537  	if sec < 0 {
1538  		// Operate on absolute value.
1539  		neg = true
1540  		sec = -sec
1541  		nsec = -nsec
1542  		if nsec < 0 {
1543  			nsec += 1e9
1544  			sec-- // sec >= 1 before the -- so safe
1545  		}
1546  	}
1547
1548  	switch {
1549  	// Special case: 2d divides 1 second.
1550  	case d < Second && Second%(d+d) == 0:
1551  		qmod2 = int(nsec/int32(d)) & 1
1552  		r = Duration(nsec % int32(d))
1553
1554  	// Special case: d is a multiple of 1 second.
1555  	case d%Second == 0:
1556  		d1 := int64(d / Second)
1557  		qmod2 = int(sec/d1) & 1
1558  		r = Duration(sec%d1)*Second + Duration(nsec)
1559
1560  	// General case.
1561  	// This could be faster if more cleverness were applied,
1562  	// but it's really only here to avoid special case restrictions in the API.
1563  	// No one will care about these cases.
1564  	default:
1565  		// Compute nanoseconds as 128-bit number.
1566  		sec := uint64(sec)
1567  		tmp := (sec >> 32) * 1e9
1568  		u1 := tmp >> 32
1569  		u0 := tmp << 32
1570  		tmp = (sec & 0xFFFFFFFF) * 1e9
1571  		u0x, u0 := u0, u0+tmp
1572  		if u0 < u0x {
1573  			u1++
1574  		}
1575  		u0x, u0 = u0, u0+uint64(nsec)
1576  		if u0 < u0x {
1577  			u1++
1578  		}
1579
1580  		// Compute remainder by subtracting r<<k for decreasing k.
1581  		// Quotient parity is whether we subtract on last round.
1582  		d1 := uint64(d)
1583  		for d1>>63 != 1 {
1584  			d1 <<= 1
1585  		}
1586  		d0 := uint64(0)
1587  		for {
1588  			qmod2 = 0
1589  			if u1 > d1 || u1 == d1 && u0 >= d0 {
1590  				// subtract
1591  				qmod2 = 1
1592  				u0x, u0 = u0, u0-d0
1593  				if u0 > u0x {
1594  					u1--
1595  				}
1596  				u1 -= d1
1597  			}
1598  			if d1 == 0 && d0 == uint64(d) {
1599  				break
1600  			}
1601  			d0 >>= 1
1602  			d0 |= (d1 & 1) << 63
1603  			d1 >>= 1
1604  		}
1605  		r = Duration(u0)
1606  	}
1607
1608  	if neg && r != 0 {
1609  		// If input was negative and not an exact multiple of d, we computed q, r such that
1610  		//	q*d + r = -t
1611  		// But the right answers are given by -(q-1), d-r:
1612  		//	q*d + r = -t
1613  		//	-q*d - r = t
1614  		//	-(q-1)*d + (d - r) = t
1615  		qmod2 ^= 1
1616  		r = d - r
1617  	}
1618  	return
1619  }
1620
```

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